It is well known that an internal latent image type silver halide emulsion not having been previously fogged is imagewise exposed to light and then subjected to surface development either after fogging treatment or while fogging treatment to obtain a direct positive image.
The term "internal latent image type silver halide photographic emulsion" uses herein means a silver halide photographic emulsion of the type in which sensitivity specks are predominantly present in the interior of silver halide grains and a latent image is predominantly formed in the interior of the grains by exposure to light.
Various techniques in this field of technology, such as those described, for example, in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, and 3,796,577, British Pat. Nos. 1,151,363, 1,150,553, and 1,011,062. According to these conventional techniques, photographic light-sensitive materials providing a direct positive image with relatively high sensitivity can be produced.
For the details of the direct positive image formation mechanism, reference can be made to it, for example, in T.H. James, The Theory of the Photographic Process, 4th. Ed., Ch. 7, pp. 182 to 193 and U.S. Pat. No. 3,761,276.
It is believed that a direct positive image is formed through the following mechanism: First, imagewise exposure results in the formation of an internal latent image in the interior of silver halide grains, which leads to the formation of fog centers selectively on the surface of the unexposed silver halide grains by surface desensitization based on the internal latent image, and subsequent conventional surface development processing results in formation of a photographic image (a direct positive image) on the unexposed area.
Selective formation of fog centers described above can be generally effected by a "light fogging method" in which the entire surface of a light-sensitive layer is secondarily exposed to light as described, for example, in British Pat. No. 1,151,363 or a "chemical fogging method" using a nucleating agent as described, for example, in Research Disclosure, Vol. 151, No. 15162 (Nov., 1976), pp. 76 to 78.
Hydrazine compounds are well known as nucleating agents which are employed in the above-described "chemical fogging method".
As another type of nucleating agents, heterocyclic quaternary ammonium salts are known and described, for example, in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Pat. Nos. 1,283,835, JP-A-52-3426, and JP-A-52-69613 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In general, the above described hydrazine type nucleating agents are most excellent in view of discrimination since they provide a large difference between maximum density (D.sub.max) and minimum density (D.sub.min). However, they are disadvantageous because they require processing at a high pH (pH&gt;12).
As nucleating agents which function in processing at a low pH (pH.ltoreq.12), heterocyclic quaternary ammonium salts as described above are known. Particularly, propargyl- or butynyl-substituted heterocyclic quaternary ammonium salt compounds as described in U.S. Pat. No. 4,115,122 are excellent nucleating agents in view of discrimination when used in direct positive silver halide emulsions.
However, in silver halide emulsions, sensitizing dyes are employed for the purpose of spectral sensitization. In such cases, competitive adsorption of the sensitizing dyes and the heterocyclic quaternary ammonium type nucleating agents onto silver halide grains takes place, and thus, it is necessary to add a large amount of the quaternary ammonium salt type nucleating agents which are of low adsorptivity. In particular, in the case of multilayer color photographic light-sensitive materials, unevenness of density and destroy of color balance may undesirably occur. Therefore, these compounds are still insufficient. Further, these tendencies become more remarkable upon preservation of the photographic material under high temperature and high humidity conditions.
In order to solve the above described problems, quaternary salt type nucleating agents having an AgX adsorption accelerating thioamido group are disclosed in U.S. Pat. No. 4,471,044. Although the amount to be added necessary to obtain a sufficiently high D.sub.max is reduced and the decrease in D.sub.max during preservation at high temperature is controlled by introduction of the adsorptive group, these effects do not still achieve to a fully satisfactory level.